This invention relates to power supplies in general, and more particularly, it relates to high frequency inverter power supplies.
It has been common practice to utilize Bipolar Switching Transistors as the switching device in prior art inverter type power supplies. Bipolar Transistors in power supplies operating in the 20 to 30 kHz range have been widely used. When it is desired to move the switching power supply operation into frequencies of 200 kHz or higher, problems with the use of Bipolar Transistors are encountered. The Bipolar Transistors are current controlled and often require ampere-level base-drive current sources. Further, they exhibit switching delays due to the minority-carrier storage time. Further, Bipolar Transistors are subject to second-breakdown failure; and, due to their negative temperature coefficient, tend to result in failure due to current crowding within the device; and tend to be sensitive to temperature variations.
The inverter type power supplies often require isolation between drive circuitry and the Power Switching device, the most common type of isolation being provided by a transformer. When it is attempted to move the operational rates into the higher frequency ranges, it would normally be required that the transformer have low bias requirements, that is low magnetizing current, low interwinding capacitance and low leakage inductance. A transformer to meet these requirements for a very fast switching application, for example in the range of 15 nanoseconds, would be very difficult and costly to design and achieve.